Concrete-lined pipes of exceptionally large diameter are generally used as buried conduits for conducting drinking water, irrigation water, and other fluids. To construct such a pipe, concrete mortar is placed inside a steel pipe which is then spun to centrifugally distribute the concrete mortar in an even thin layer on the inside wall of the pipe.
A machine, generally known as lining machine, is used to perform the operation and basically consists of multiple belt assemblies, usually three to five, which are spaced apart for supporting the pipe along its length. A side view of a pre-existing lining machine 10 is illustrated in FIG. 1. The side view shows the typical belt arrangement for each belt assembly of the lining machine. Each belt 12 is formed into a loop which wraps around a set of four pulleys, as depicted in FIG. 1.
The pipe is supported by the belts at the portion between the two upper pulleys 14, 16. The left upper pulley 14 is generally fixed in position and powered by a drive motor to provide the necessary drive force to spin the pipe at sufficient rotational speeds to adequately pack the mortar on the inside wall of the pipe. The right upper pulley 16 can be adjusted toward or away from the left pulley before the spinning operation to adapt the machine to a range of pipe sizes. The right lower pulley 20 may also be adjusted. The left lower pulley 18 is fixed in position. All pulleys are fixed in position during operation.
To mortar line a pipe, the pipe is initially rotated at a steady but relatively low speed and a mortar feeding lance is moved inside the pipe to pour the mortar material along the length of the pipe. The pipe is then accelerated to a desired rotational speed to pack the mortar against the internal pipe wall for a sufficient period of time to allow the mortar to dewater.
Due to the elastic property of the belts, the generally uneven roundness of the pipe, and the imbalance in the pipe caused by an uneven mortar thickness caused by the rotation of the pipe around its mass (i.e., gravity) center, the pipe vibrates on the belts while it is rotating. The pipe and the belts together constitute an oscillating system with a particular frequency of its own, its so-called "natural frequency." When the pipe is rotated at high speeds, the reciprocating movements of the belts come into the natural frequency range of this system. When this happens, the pipe and belts tend to move independently of the motion imparted to them by the drive motor.
The vibration of the system is especially large when in the natural frequency of the system before reaching the packing speed. If the pipe is excessively out of balance, the vibration can be very severe and the pipe can bounce off of the belts, causing the belts to slack and sometimes slip away from the pulleys. This damages the belts and may also cause the pipe to fall off of the machine, thereby creating a dangerous situation for both the equipment and human operators of the lining machine.
Accordingly, it would be desirable to provide means for dampening the vibration of the system during the spinning operation for the safety of the machine and its operators.